Structural MRI may have potential as a biomarker for both pre-symptomatic and symptomatic clinical trials for HD, because there is a wealth of evidence that striatal atrophy can be detected using MRI up to 23 years prior to predicted motor onset and that striatal volume is negatively correlated with motor and cognitive function and CAG repeat length, and continues to shrink along with disease progression. Thus by the time individuals with HD gene expansion show clinical symptoms, striatal volumes are reduced by more than half. Neuroprotective therapy might provide a more effective benefit if given before overt clinical symptoms. The challenge is to evaluate putative neuroprotection in the absence of clinical symptoms, therefore, the development of MRI measures as biomarkers for HD could be extremely valuable for the assessment of neuroprotective therapies. Validation of these measures for clinical trials, however, requires a demonstration that they change with treatment. RNA interference (RNAi) is emerging as a powerful candidate strategy for treating HD, perhaps potentially more effective than current pharmacological candidates. AAV-miRNA (AAV-mi2.4) was demonstrated to be safe, and provided therapeutic benefit in N171-82Q HD mice. We have recently characterized brain volumetric changes longitudinally along with disease progression in mouse models of HD. In the current proposal, we aim to validate structural MRI measures as biomarkers in response to RNAi treatment and further evaluate therapeutic efficacy of silencing mutant huntingtin by using MRI measures in tandem with behavioral and pathological assessments in an HD mouse model. Specific Aim 1: We will determine whether longitudinal MRI measurements can detect efficacy of AAV-mi2.4 on brain atrophy in N171- 82Q HD mice. Baseline brain volume will be obtained by MRI. AAV-mi2.4 (RNAi) or AAV-hrGFP will be injected into mouse striatum and MRI scans combined with unbiased automatic whole brain segmentation analysis will be used to evaluate volumetric changes of whole brain, striatum, and neocortex longitudinally. Mice will be perfused for histological analysis. Motor function and body weight will be assessed at same time points. Specific Aim 2: We will determine the efficacy of a duplex allele-specific mutant huntingtin siRNA (PM3) by structural MRI in N171-82Q HD mice. We hypothesize that allele-specific RNAi will provide beneficial effects that can be detected by MRI measures in HD mice. AAV-allele-specific RNAi will be injected into mouse striatum. MRI scans will be performed longitudinally. Correlation between histology data and MRI data will be analyzed. Motor behavioral performance and body weight will be assessed at the same time points. The goal of this study is to provide proof-of-principle for structure MRI as a biomarker to evaluate therapeutic efficacy in preclinical studies. The ultimate goal is to provide a rationale for the use of structural MRI and potentially RNAi strategy in therapeutic trials of human HD for FDA approval. PUBLIC HEALTH RELEVANCE: Neuroprotective therapy might provide a more effective benefit if given before overt clinical symptoms. The development of MRI measures as biomarkers for HD could be extremely valuable for the assessment of neuroprotective therapies. The goal of this study is to provide proof-of-principle for structure MRI as a biomarker to evaluate therapeutic efficacy in preclinical studies.